Super XGA is 1400x1050
Ultra XGA is 1600x1200
Ultra Sharp UXGA is 1600x1200
The Ultra Sharp has a higher contrast, viewing angles, and color depth than the standard UXGA. Dell has also called this a "Enhanced UXGA".
I am pretty sure Dell has info on this on their order page.. a "help me choose" link or something.
FYI, I have the regular UXGA and love it.. I bought it before the US UXGA came out.